1. Field of the Invention
The present invention relates to a laser device of a silent discharge type in which silent discharging is used to excite laser oscillation.
2. Description of the Prior Art
Referring to FIG. 4, there is shown a CO.sub.2 laser device in which silent discharging is used to promote laser oscillation. More specifically, FIGS. 4 (a) and (b) are front and side views of the laser device showing its general arrangement wherein when an output of an AC power source 1 is applied between a pair of electrodes 2, 3 coated with dielectric material, silent discharge takes place in a discharging space 4 defined by the electrodes and acts to excite laser medium gas 5 trapped in the space 4. As a result, laser oscillation occurs in an optical resonator which comprises a totally-reflective mirror 6 and a semi-transparent (that is, a partially-reflective) mirror 7. Such laser oscillation is partially taken out as a laser beam through the semi-transparent mirror 7.
The above-mentioned laser medium gas is passed through the discharging space 4 at a velocity of several tens to several hundreds m/s by blowers 8, cooled by a heat exchanger 9 and sent again to the discharging space 4.
Shown in FIG. 5 is an electrical equivalent circuit of the foregoing laser device in such a condition that silent discharge is occurring between the electrodes 2 and 3. In the drawing, reference numerals 10 and 11 represent the capacitances of the dielectric-coat layers of the electrodes 2 and 3 respectively, 12 the capacitance of a gap between the electrodes 2 and 3, 13 the equivalent resistance of silent discharge zone of the space 4, 14 a stray capacitance present in wiring lines and a chamber 15. FIG. 6 is an equivalent circuit when the capacitances 10 and 11 in FIG. 5 are combined to form a single capacitance 16.
These equivalent circuits have been found by the inventors of the present invention as a result of their studies, and it has been found that such equivalent circuits are very useful in explaining the discharging characteristic of the laser device in the laser medium gas when the frequency of the power source 1 exceeds 10 KHz.
More specifically, FIG. 7 shows a laser oscillation characteristic, that is, a relationship between a silent discharging power W.sub.d (input power) consumed by the equivalent resistance 13 and a laser output W.sub.r. It will be seen from FIG. 7 that a constant laser output can be obtained by controlling the discharge power W.sub.d to be constant.
Assuming that the values of the capacitances 12, 14 and 16 are C.sub.g, C.sub.m and C.sub.d respectively and the value of the equivalent resistance 13 is R in FIG. 6, vector relationships between the current and voltage at various points in FIG. 6 are as shown in FIG. 8 wherein physical quantities with dots are in the form of complex while physical quantities without dots are in the form of absolute value (effective value when phase is not taken into consideration).
From FIG. 8, the discharging power W.sub.d is expressed by: EQU W.sub.d =V.sub.d .multidot.I.sub.d. . . (1)
where, V.sub.d is a terminal-to-terminal voltage across the equivalent resistance 13 and I.sub.d is a current flowing through the resistance 13. It will be appreciated from the equation (1) that V.sub.d is a constant, determined by the discharging conditions and thus when the current I.sub.d is made constant, the discharging power W.sub.d, i.e., the laser output can be made constant.
Referring to FIGS. 9 and 10, there are shown other prior-art laser devices in which an output voltage V.sub.a of a power source 1 is controlled so that an output current I.sub.a of the power source 1 detected by respective current detectors 17 and 18 is equal to a set value, as proposed, for example, in Japanese Patent Laid-Open Publication No. 147185/83. However, these laser devices are defective in that it is impossible to detect a variation in the current I.sub.d resulting from a variation in the stray impedance (based on the capacitance C.sub.m in FIG. 6) present in lines wired between the power source 1 and the laser oscillation section or resulting from a variation in the capacitance C.sub.d, thus impeding the sufficient supression of variation of the discharging power W.sub.d.
Shown in FIG. 11 in a vector representation is a relationship between the current and voltage at the various points of FIGS. 9 and 10 when the above-mentioned capacitance C.sub.d is varied.
Generally, the silent discharge current I.sub.b shown in FIG. 6 contains, as shown in FIG. 12, such pulse current components that are considerably high in frequency than the power source 1 and that vary irregularly. In such a case, the foregoing prior art devices have had a problem that it is difficult to detect reliably the above-mentioned pulse current due to the frequency characteristics of the current detectors 17 and 18, resulting in a poor controllability.
Accordingly, the present invention is directed to eliminating such problems in the prior art. To this end, in accordance with the present invention, there are provided a means for detecting the amount of electric charges passed between electrodes and a means for controlling an output voltage or current of a power source so that the detected value of the charge-amount detecting means is equal to a present value. The present invention can control its discharging power to be constant regardless of variations in the capacitances of dielectric material layers coated on the discharge electrodes or variations in the capacitances present in the wiring lines and chamber. Further, since the charge amount is accurately detected while not affected by such pulse current, the present invention can provide a high control accuracy.